1. Field of the Invention
This invention relates generally to charged-particle beam apparatus, and in particular to an improved electron beam optical apparatus for irradiating a specimen in a two-dimensional pattern having first areas which are unexposed to the electron beam and are surrounded at least almost entirely by second areas which are exposed to the electron beam.
2. Description of the Prior Art
It is known in the art to generate two-dimensional patterns of the above-described type by means of a scanning electron microscope. In such an apparatus, the electron beam of the microscope is focused on a specimen and guided thereover in raster fashion. Light and dark modulation of the electron beam is accomplished in one known apparatus by means of a "flying spot scanner." Such a scanner comprises a cathode-ray tube fluorescent screen which is scanned uniformly by the electron beam. A light spot results and illuminates a mask having a pattern corresponding to a two-dimensional pattern which is to be generated. The light passed by the mask serves as an unblanking signal for the scanning microscope which is synchronized with the cathode-ray tube. Further details of this apparatus are described in 44 Rev. Sci. Instr. 1282-1285 (1973).
In another known apparatus, the electron beam of the microscope is unblanked by means of a digital computer. In this device, the electron beam is sequentially guided in raster fashion across the areas to be exposed. Further details of this apparatus are described in 10 J. Vac. Sci. Technol. 1052-1055 (1973).
The two aforementioned unblanking apparatus have the disadvantage that they are considerably complex in construction. The time required to form a two-dimensional pattern, however, depends upon the shape and size thereof, and is at least equal to the product of the total number of raster dots to be exposed to the electron beam of the apparatus and the required exposure time of each dot. (A raster "dot", as used herein, refers to a screen area which is equal to the cross-sectional area of the electron beam at the specimen examined).